Implement `std::fmt::Debug` for `ecs::observer::Trigger`

[Sorseg]

Objective

I tried writing something like this in my project

.observe(|e: Trigger<OnAdd, Skeleton>| {
    panic!("Skeletoned! {e:?}");
});

and it didn't compile. Having Debug trait defined on Trigger event will ease debugging the observers a little bit.

Solution

Add a bespoke Debug implementation when both the bundle and the event have Debug implemented for them.

Testing

I've added println!("{trigger:#?}"); to the observers example and it compiled!

Caveats with this PR are:

• removing this implementation if for any reason we will need it, will be a breaking change
• the implementation is manually generated, which adds potential toil when changing the Trigger structure

Showcase

Log output:

on_add_mine: Trigger {
    event: OnAdd,
    propagate: false,
    trigger: ObserverTrigger {
        observer: 2v1#4294967298,
        event_type: ComponentId(
            0,
        ),
        entity: 454v1#4294967750,
    },
    _marker: PhantomData<observers::Mine>,
}

Thank you for maintaining this engine! :orange_heart:

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[mnmaita]

Looks like we added (almost) the same labels at the same time 😅 @MrGVSV

[Sorseg]

I can fix the doctest some time tomorrow

[Sorseg]

Doctests for bevy_ecs are run separately, bevy_ecs doesn't depend on bevy, meaning it doesn't have access to the DefaultPlugins.

I tried to use the minimal startup_system example, but bevy_ecs doesn't have access to bevy_app either.

I think the doctest in its current form does not provide value, as it is impossible to make it compile while still looking as regular bevy code. I'll remove it